Production of biphenyltetracarboxylic acid tetraesters, whereby a phthalic acid diester is subjected to oxidative dimerization reaction in a molecular oxygen-containing atmosphere at high temperature in the presence of a catalyst containing a palladium compound, is known. It is a drawback of this process, however, that the palladium compound-comprising catalyst is easily deactivated during the reaction, and various attempts have been made to overcome this drawback.
Japanese Unexamined Patent Publication SHO No. 60-51150 discloses a process for the production of biphenyltetracarboxylic acid esters wherein an ortho-phthalic acid ester is oxidatively coupled by supplying a molecular oxygen-containing gas to the reaction system at high temperature and in the presence of a catalyst comprising a palladium salt, a basic bidentate ligand and a copper salt, and is then further oxidatively coupled by successive addition of the catalyst component to the reaction system 1-10 times.
Japanese Unexamined Patent Publication SHO No. 61-106541 discloses a process for production of biphenyltetracarboxylic acid esters wherein a phthalic acid ester is oxidatively coupled while continuously or intermittently supplying a β-diketone to the reaction system at high temperature in a molecular oxygen-containing atmosphere in the presence of a catalyst comprising a palladium salt and a copper salt.
Japanese Unexamined Patent Publication SHO No. 64-48 discloses a process for production of biphenyltetracarboxylic acid tetraesters by oxidative dimerization of an ortho-phthalic acid diester in the presence of a catalyst comprising a palladium salt, a basic bidentate ligand and a copper salt, wherein the reaction mixture is circulated through the circulation line, and, the catalyst component is introduced through the circulation line of the reactor, to continuously replenish the catalyst component to the reactor.
Japanese Unexamined Patent Publication No. 2000-186063 discloses a process for production of 3,3′,4,4′-biphenyltetracarboxylic acid tetraesters whereby an ortho-phthalic acid diester is dimerized by successive addition of a catalyst component comprising powdered palladium salt or compound with a specific surface area of 0.5 m2/g or greater and a basic bidentate ligand compound.
All of these processes involve successive addition of the catalyst. Successive addition of the catalyst in these processes maintains catalytic activity for increased reaction efficiency. However, the processes are associated with several problems, including the requirement for special equipment, complicated procedures for successive catalyst addition, and difficulty in controlling the reaction because of rapid variation of the catalyst concentration(s) in the reaction mixture during addition of the catalyst(s), whereby improvement in reproductivity of a high reaction yield and a high selection of a target product is desired. Further, when the catalyst is successively supplied in large amounts during continuous prolonged reaction, greater production of by-products through reaction between the target product and the starting materials occurs, whereby reproductivity of a high reaction yield and a high selection of a target product for a long time period is desired.
In order to solve the problem of easy deactivation of catalysts comprising palladium compounds, additives which maintain the catalystic activity for longer periods have been investigated. Japanese Unexamined Patent Publication SHO No. 64-56649 discloses a process for production of biphenyltetracarboxylic acid esters wherein an ortho-phthalic acid diester is heated while supplying a molecular oxygen-containing gas to the reaction system in the presence of a catalyst comprising a divalent palladium salt, a copper salt and a basic bidentate ligand, wherein deactivation of the catalyst is limited by adding perchloric acid to the reaction system. However, the process cannot be considered practical because of the problem of risk of explosion involved in using perchloric acid.
Moreover, all of the production processes in the aforementioned patent publications are based on batch process. As the production processes therefore require a complicated procedure of heating or cooling at each start or termination of the reaction, they are also problematic in economical terms due to energy loss from heating and cooling and unavoidable material loss with opening of the apparatus for each loading during the batch operation, and the processes are therefore in need of improvement. However, none of the aforementioned patent publications have described concrete means or methods for improving continuous production process.
It is an object of the present invention to provide an improved economical process for production of biphenyltetracarboxylic acid tetraesters whereby a phthalic acid diester is subjected to an oxidative dimerization reaction at high temperature while supplying molecular oxygen to the reaction mixture in the presence of a catalyst containing a palladium compound, which process requires no special equipment or complicated procedures and allows effective utilization of the catalyst for easy control of the reaction and continuous production.